This invention relates to measuring the concentration of hydrogen or other reducing compound in a gaseous mixture using a zinc oxide thin film sensing element. More particularly, this invention relates to a zinc oxide thin film semiconductor having a gas-sensing surface that is partially covered by a platinum-gold alloy to substantially improve the responsiveness to reducing gas.
It has been reported that the electrical resistance of a semiconductive body of tin oxide or zinc oxide exposed to gas is sensitive to hydrogen or other reducing compounds in the gas. Also, it has been reported that the addition of palladium, platinum or gold enhances the sensitivity of the oxide body. For example, in U.S. Pat. No. 4,030,340 issued to Chang, June 21, 1977, a discontinuous coating of palladium or pallidum-gold alloy on a sputtered tin oxide thin film improved hydrogen sensitivity. Both coating materials performed substantially the same. In contrast, I have found that a zinc oxide thin film produced by sputtering from a sintered zinc oxide target does not respond to hydrogen. Neither is the film sensitized to hydrogen by a discontinuous coating of palladium metal or gold metal alone. Thus, it is totally unexpected that an alloy of the two metals would cause the film to be hydrogen sensitive.
Therefore, it is an object of this invention to provide a sensor and a method employing the sensor for detecting the presence of hydrogen, alkenes or other reducing species in a gaseous sample. The sensor comprises a sputtered zinc oxide thin film semiconductor whose electrical resistance alone is relatively insensitive of the reducing species. The sensor also comprises a gold-palladium alloy located so as to interact with the mixture and with the film in a manner that causes the film resistance to be affected by the reducing species.
It is a further object of this invention to provide a sensor and a method for using the sensor for detecting hydrogen or alkenes in a gaseous sample, which sensor comprises a thin film composed of a sputtered zinc oxide material and having islands of a palladium-gold alloy distributed on the surface. The palladium and gold synergistically cooperate to substantially improve the sensitivity of the zinc oxide resistance to the presence of the reducing species.